Bleaching of groundwood-sulfite pulp mixtures



Aug. 31, 1954 F'. L. FENNELL 2,687,942

BLEACHING 0F GROUNDWOOD-SULFITE PULP MIXTURES Filed March 19, 1951 OPTIMUM ALKALINITIES FOR BLEACHING MIXTURES OF SULFITE AND GROUNDWOOD PULPS A EXPECTED OPTIMUM ALKALINITIES B OPTIMUM ALKALINITIES ALKALINITY AS IO 20 3O 4O 5O 6O 70 8O 9O 'IOO "A GROUNDWOOD IN PULP MIXTURE FRANCIS L. FENNELL INVENTOR.

AGENT.

Patented Aug. 31, 1954 BLEACHING OF GROUNDWOOD-SULFITE PULP MIXTURES Francis L. Fennell, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application March 19, 1951, Serial N 0. 216,343

10 Claims.

This invention relates to the bleaching of mixtures of groundwood and sulfite wood pulps.

It is frequently desirable to use as furnishes for printing papers mixtures of groundwood or mechanical wood pulp and sulfite wood pulp. For such uses the groundwood and sulfite pulps may be separately belached and then mixed in the desired proportions. However, the use of mixtures of these pulps becomes especially attractive when the desired mixtures can be bleached in a single bleaching installation rather than in separate installations. It has been found that bleaching conditions which are optimum for either groundwood or sulfite pulp separately are not optimum for mixtures of these pulps, and that the optimum conditions for such mixtures are not obvious from the optimum conditions for bleaching the two pulps separately. The present invention is concerned with a method for bleaching mixtures of these pulps whereby improved and optimum bleaching results are obtained.

One object of the present invention is to provide an improved method for bleaching mixtures of groundwood and sulfite wood pulps. Another object is a method for bleaching such mixtures wherein the alkalinity of the bleach liquor is adjusted to a value which is substantially lower than that which would be expected to be optimum from a consideration of the optimum alkalinities for bleaching either type of pulp alone. Still further objects will be apparent from the following description.

The above objects are accomplished in accordance with the invention by bleaching a mixture of groundwood and sulfite wood pulps containing a substantial proportion of each of said pulps, with an alkaline peroxide bleach liquor having an alkalinity substantially less than that which, would be expected to be optimum for bleaching said mixture from a consideration of the optimum alkalinities for bleaching said pulps alone. It has been discovered that whereas bleaching conditions other than alkalinity appear not to be critical, alkalinity is a critical factor with the optimum alkalinity for any given mixture being outstandingly lower than would be predicted from the responses of the individual pulps.

All alkalinities reported hereinare expressed as percent by weight NaOH based on the moisture-free weight of the pulp being bleached. Furthermore, the alkalinities given are the initial alkalinities of the bleach liquor, e. g., at the beginning of the bleaching operation, and do not represent the alkalinity at the end or at any other stage of the bleaching process. A substantial drop in alkalinity generally occurs during the bleaching reaction. In expressing alkalinity in terms of sodium hydroxide, it is not to be understood that sodium hydroxide is the only alkaline material that can be used, since any suitable commonly used alkalizing agent maybe employed to obtain the required alkalinity. As a convenient method for determining the alkalinity of the bleach liquor, 5 cc. of the liquor is diluted to cc. and then titrated with 0.1 N H2804 to a yellow straw color using Phenol Red as indicator. The amount of NaOH equivalent to the acid titer is calculated, and from the value obtained and from the proportion of bleach liquor to pulp to be used, the alkalinity of the bleach liquor expressed as percent NaOH based on the weight of the dry pulp is readily calculated.

One preferred method for practicing the invention is to subject a pulp mixture containing from about 10 to 90% groundwood and 90 to 10% sulfite pulp to the action of an alkaline peroxide bleach liquor having an alkalinity equal to about 1.4% NaOH based on the dry weight of the pulp at a temperature of F. and at a pulp consistency of 12%. Bleaching will be continued under these conditions for about 6 hours or until the residual peroxide content of the slurry has dropped to about 15 to 20%, after which the pulp may be thoroughly washed to free it of residual peroxide. If desired, the bleach pulp may be treated with a reducing agent such as sulfur dioxide or the like to destroy residual peroxide. Reducing agents such as sulfur dioxide,- sodium bisulfite, sodium metabisulfite, sodium thiosulfate and sodium hydrosulfite are suitable for this purpose.

The invention is further illustrated by the following examples:

EXAMPLE 1 Numerous bleaching trials were carried out at a pulp consistency of 6% and a temperature of 120 F. The aqueous bleach liquor used contained 0.025% Epsom salt, 5.0% of a 41.5 B. sodium silicate solution (containing 8.9% NaaO, 290% S102 and 62.1% H2O based on the weight of the silicate solution) and 2.0% sodium peroxide. These percentages of Epsom salt, silicate solution and sodium peroxide were based on the dry weight of the pulp. In each trial, there was added to the bleach liquor prior to mixing with the pulp that amount of sulfuric acid required to leave in the solution the amount of alkali desired for the trial. The bleaching reaction was terminated in each case when the peroxide content of the system fell to about 20% of its original value, or at the end of 6 hours if the residual peroxide content had not fallen to 20% at the end of that time. The reason for so terminating bleaching is that it is generally impractical to continue bleaching for a time longer than about 6 hours or after the residual peroxide content has reached 15 to 20%.

Bleaching was terminated in each of the above trials by treating the pulp with a dilute (45 g. per liter) solution of sodium metabisulfite at a pH of 7.0. The pulps were then acidified with all cases under comparable conditions using a' General Electric reflectometer which measures reflectance on a scale ranging from to 100. Comparative brightness values 'were'also determined for the unbleached pulps .and pulp mixtures using hand sheets prepared from pulp slurries adjusted to a pH of 4.5Twith sulfuric acid.

The results obtained are tabulated below. In the. table, the optimum alkalinities are indicated by asterisks. For each of the -pulps containing 0, 40; 50 and 100% groundwood,-several trials areshown employing alkalinities ranging'from about 1.0 to 2.2% NaOH. Similar trials were also run for the'other pulp mixtures shown, but for the sake of brevity, only optimum alkalinities are given in the table. In determining the optimum alkalinitiesfor a specific mixture, the brightness-gains found in the several trials were plotted against the corresponding alkalinities and frbmthe resulting curve the alkalinity corresponding to maximum brightness gain was sele'cted as the optimum'alkalinity.

Bleachingresults (Example 1) portion of 'groundwoodidecreasess or-theiproportion of sulfite increases. The results represented by curve B demonstate that for bleaching any mixture of groundwood and sulfite pulps the opti- 5 mum-amount of alkali to be employed in the bleach liquor will be less than the optimum amount for either pulp alone.

EXAMPLE 2 A pulp mixture containing 60% groundwood was efiected at 'a=pu1p-consistency of 12%; The

brightness gains obtained'were 6.'2,6.0, 8.'-1and 716, respectively, when using-bleach liquors 'having initial'alkalinities equivalent'to 1.89, 1.49; 1.24 and 0.99% NaOH based'on the dry weight of the pulp. The sources of'the individual pulps for this mixture'were difierent from the sources for the individual pulps of Example 1. As indicated by the lower brightness gains, the pulp mixture for this example was-more difficult to bleach than were'the mixtures of Example 1.

Nevertheless, it is obvious from the above results that bleaching was substantially more effective at Pulp Mixture (percent by wt.) Alka P t Brightness llnlty as 'lnal Percent Hrs. Resui.

Ground" Sulflte N Peroxide 0:1 Final Gain wood o 100 2. 22 4 23 54. 2 65. 6 11. 4 o 100 '2. 06 54. 2 67.0 12.8 0 100' 1.81 6 53 54. 2 66.8 12.6 0 100' D. 99 6 52 54. 2 65. 1 10. 9 15 85 '1. 45 54. 2 68. 4 14. 2 40 6o 2. 22 2 21 55. 3 64. 9 8.6 40 66' 1. 49 6 so 56. 3 6s; 6 12. 3 4o 60 '1. 41 56. a 68. 6 12. 3 4o 60 1. 24 6 33 55. 3 68. 5 12. 2 4o 60' 0.119 6 45 56. 3 67. 1 10. s 56 5o 2. 22 1 15 60. 7 71. 6 1o. 9 5o 1. 49 2 20 6o. 7 73. 4 12. 7 50 59 '1. 41 60. 7 73. 6 12. 9 50 5o 1. 24 4 19 60.7 76. 6 12. 9 50 59 0. 99 6 19 6o. 7 73.0 12. 5 7o 30 '1; 41 60.6 71.5 10.9 90 10 1. 41 60.2 71. 7 10. 5 100 o 2. 22 0. 5 19 62.4 71.4 9. 0 100 o 1. o. 5 21 62. 4 71.7 9. a 100 0 '1. 49 1 25 62. 4 72.8 10. 4 100 o 1. 24 4 17 62.4 71.8 9. 4

Optimum alkalinity;

The optimum alkalinities shown above have been plotted against the 'groundwood content of the pulp mixtures in the accompanying drawing. The resulting curve B representing optimum alkalinities should be compared with broken line A. on the drawing which represents the alkalinities which would be expected to be optimum from consideration of the optimum alkalinities for bleaching the two types of pulp separately.

Referring to the drawing it will be seen that the optimum alkalinity for mixtures containing from :around 25 to slightly over 90% groundwood is around 1.4% NaOH whereas the expected optimum alkalinities for these mixtures as indicated by broken line A would be within the range of about 1.92 to about 1.54%. It is further obvious from curve B that. the optimum alkalinity for a umixture containing even as little as 10% groundwood is much lower than the optimum alkalinity for bleaching 100% sulfite. It is also evident that within the range of from about 10% to about 90% groundwood, the difference between. the actual optimum and the expected optimum alkalinityincreases substantially as the prothe lower than at the higher alkalinities-shown. Improved bleachingresultshave also been ob-.' tained at such lower alkalinities when bleaching similar mixtures at a pulp consistencyof 20%.

While the alkalinity or alkali content of the bleach liquor has been expressed herein in terms of its NaOH equivalent,. it should be understood that the alkalinity of such liquors may be due in part or entirely to the presence-of alkaline agents other than sodium hydroxide. Any of the usual alkalizing agents can be used, including other alkali metal and alkaline earth metal hydroxides and'peroxides, and the alkali metal si1i-. cates, borates, carbonates and phosphates, e. g., sodium pyrophosphate and trisodium. phosphate. It is preferred however that the required alkalinity be due at least in major partto sodium hydroxide added as such or assodium peroxide.

It is also preferred that thebleach bath contain a substantial amount ofv sodium silicate, e. g., 3 to 5% based on the dry weight of the pulp; since the presence of. silicate is well known to be advantageous in. alkaline peroxide pulpbleaching solutions. It is-also' advantageous to include'in;

the bath a stabilizer such as Epsom salt, e. g., in an amount ranging from about 0.01 to 0.5%.

Substantial improvement in bleaching results will be obtained in accordance with the invention when a groundwood-sulfite pulp mixture is bleached with an alkaline peroxide bleach liquor having any alkalinity which is substantiallylower than the expected optimum alkalinity for that pulp mixture as indicated from the optimum alkalinities for bleaching the individual pulps. Ordinarily, alkalinities in the range 0.8 to 1.5% NaOH will be chosen. At alkalinities below about 0.8% bleaching generally occurs at a rate too slow to be practical while at above about 1.5% bleaching becomes generally inefficient. The most preferred alkalinity will vary somewhat with the sources of the pulps but usually will be within the range 1.2 to 1.45% NaOH.

Bleaching conditions other than alkalinity appear not to be critical, all indications being that substantial advantages will result from the use of alkalinities within the ranges stated above regardless of the bleaching temperature, peroxide concentration or pulp consistency used. Accordingly, any temperature, peroxide or active oxygen concentration, or pulp consistency suitable for bleaching either groundwood or sulfite pulp alone may be used. Temperatures within the range 80 to 150 F. preferably 100 to 130 F., are illustrative of suitable temperatures. Bleach liquors containing active oxygen in amounts equivalent to 0.2 to about 2.5% H202, preferably 0.4 to 1.1%, based on the dry weight of the pulp generally will be used. Hydrogen peroxide, sodium peroxide, perborates, percarbonates, perphosphates or any other peroxygen compound known to be the equivalent of hydrogen peroxide in aqueous alkaline bleach baths can be used as the source of active oxygen. While good results can be obtained at any pulp consistency used in pulp bleaching processes, consistencies ranging from about to 35% appear most practical. Optimum bleaching time will depend upon the temperature, peroxide concentration, consistency and alkalinity used. Under theoptimum conditions indicated above, effective bleaching will usually result within about 6 hours or less, but a longer time can be used if desired.

It is not entirely clear why lower alkalinities give improved results when bleaching mixtures in accordance with the invention. One possible explanation is that at the optimum alkalinity for bleaching groundwood, or at higher alkalinities, the groundwood bleaching reaction proceeds much more rapidly than the sulfite bleaching reaction, so that when bleaching mixtures of the two pulps at such alkalinities the groundwood component consumes a disproportionately large amount of the peroxide initially available. With most of the peroxide being consumed by the groundwood, the sulfite constituent remains in a relatively unbleached state. On the other hand, it appears that lowering the alkalinity in accordance with the invention tends to equalize the rates of peroxide consumption by the two constituents of the mixture, resulting in more effective and eflicient bleaching. But regardless of theory, the facts are that mixtures of the two pulps can be bleached by the present method with outstanding improvement in bleach results.

I claim:

1. The method of bleaching a wood pulp mixture containing at least 10% groundwood and at least 10% sulfite pulp, said method comprising treating said mixture at a pulp consistency of 5 to 35% with an alkaline peroxide solution having an initial alkalinity equivalent to 0.8 to 1.5% NaOH by weight based on the dryweight of said pulp mixture, and removing spent bleach liquor in liquid form from the treated pulp mixture.

2. The method of claim 1 wherein the alkalinity of said peroxide solution is equivalent to 1.2 to 1.45% NaOH.

3. The method of claim 1 wherein the treatment with the peroxide solution is effected at a temperature within the range to 150 F. employing a peroxide solution containing active oxygen equivalent to 0.2 to 2.5% H202 by Weight based on the dry weight of the pulp mixture.

4. The method of claim 3 wherein the treatment with the peroxide solution is effected at a pulp consistency of about 6 to 20%.

5. The method of claim 2 wherein the treatment with the peroxide solution is efiected at a temperature of 80 to 150 F. employing a peroxide solution containing active oxygen equivalent to 0.2 to 2.5% H202 by weight based on the dry weight of the pulp mixture.

6. The method of claim 5 wherein the treatment with the peroxide solution is effected at a temperature of to F.

'7. A method of bleaching a wood pulp mixture containing at least 10% groundwood and at least 10% sulfite pulp which comprises treating said mixture at a pulp consistency of 5 to 35% and at a temperature within the range 80-150 F. with an alkaline peroxide solution having an initial alkalinity equivalent to 0.8 to 1.5% NaOH by weight based upon the dry Weight of the said mixture, said alkalinity being due at least in major part to a substance chosen from the group consisting of sodium hydroxide, sodium peroxide and mixtures thereof with each other.

8. The method of bleaching a mixture of groundwood and sulfite wood pulps comprising treating said mixture at a pulp consistency of 5 to 35% with an alkaline peroxide solution having an initial alkalinity equivalent to 0.8 to 1.5% NaOH by weight based upon the dry weight of said mixture, and removing spent bleach liquor in liquid form from the treated pulp mixture.

9. The method of claim 1 wherein the alkalinity of the peroxide solution is equivalent to about 1.4 to 1.5% NaOH.

10. The method of claim '7 wherein the treatment with the peroxide solution is effected at a pulp consistency of about 6 to 20%.

References Cited in the file of this patent UNITED STATES PATENTS Name Date McEwen June 6, 1950 OTHER REFERENCES Number Paper 

1. THE METHOD OF BLEACHING A WOOD PULP MIXTURE CONTAINING AT LEAST 10% GROUNDWOOD AND AT LEAST 10% SULFITE PULP, SAID METHOD COMPRISING TREATING SAID MIXTURE AT A PLUP CONSISTENCY OF 5 TO 35% WITH AN ALKALINE PEROXIDE SOLUTION HAVING AN INITIAL ALKALINITY EQUIVALENT TO 0.8 TO 1.5% NAOH BY WEIGHT BASED ON THE DRYWEIGHT OF SAID PULP MIXTURE, AND REMOVING SPENT BLEACH LIQUOR IN LIQUID FORM FROM THE TREATED PULP MIXTURE. 